1. Field of the Invention
This invention concerns a expandable AC power supply device. More specifically, it concerns a expandable AC power supply device which can supply AC power for a load from batteries which are built into an uninterruptible AC power supply ("UPS") or into a portable AC power supply.
2. Description of the Related Art
An uninterruptible AC power supply device has a fixed AC power capacity and time for which it can provide backup. The user checks the AC power requirement of the OA device being used and for which backup power needs to be provided, and uses an uninterruptible AC power supply device which has the desired rating. A portable AC power supply is used when an electrical device (or load) is to be used outdoors, or in another place where it cannot be supplied directly from commercial AC power. The user charges the built-in battery ahead of time, and the battery supplies AC power to the load when the electrical device is used. Portable AC power supplies also are chosen so that their rating and capacity match the load.
Existing AC power supply devices of the type described above have several problems. For example, sometimes more OA devices are added or a device is upgraded after an uninterruptible AC power supply device has been selected and put into use, thus increasing the load which must be backed up. It is also possible that the calculation of the capacity was wrong to begin with, so that the uninterruptible AC power supply is unable to handle even the existing load. In these cases, the user finds that he must purchase a new uninterruptible AC power supply with a larger capacity. To obviate such problems, the user might choose to use a large AC power supply with more capacity than he needs. In either case, the solution is uneconomical. Also, since the size of an AC power supply increases with its capacity, using an AC power supply with a capacity which is higher than needed causes the entire unit to be larger than it needs to be.
Considering the AC power supplies which are actually available, it is clear that the range of capacities available is limited by the number of models produced. Since the increment in capacity from one model to the next will necessarily be a large one, many users have no choice but to select an AC power supply with a larger capacity than they need.
If a malfunction occurs in some part of an AC power supply, AC power cannot be supplied to the load even though the remaining capacity of the batteries may be sufficient to provide backup AC power. If such a malfunction occurs while the batteries are being used to provide backup AC power, an unforeseeable breakdown will occur which may cause damage to the load or lead to unit error.